Protein arginine methyltransferases (PRMTs) are a relatively new type of chromatin-modifying enzymes that catalyze the methylation of specific arginine residues in histone and nonhistone substrates. Aberrant expression of PRMTs has been observed in various human diseases. However, the biological impact of protein arginine methylation and the molecular basis of PRMT catalysis are poorly defined. Our long-term research goal is to elucidate the biochemical pathways mediated by key PRMTs that contribute to the pathogenesis of cancer and cardiovascular disorders, and to develop effective PRMT inhibitors. In this proposal, we plan to carry out studies on mechanism, regulation, and function of key PRMT enzymes, by exploring and applying new chemical biology approaches. Two specific aims will be pursued: a) Design unique biochemical assays to elucidate the mechanism of substrate specificity regulation by PRMT1. The mechanisms that govern the substrate specificity of PRMTs are not well understood. It also remains to determine the dynamic regulation of arginine methylation. We will introduce environmentally sensitive fluorophores to probe how key motifs in the substrates dynamically regulate arginine recognition and methylation. Also, we will use expressed protein ligation to site specifically label PRMT1 with biophysical probes to investigate the conformational changes of PRMT1 during the catalytic process. Further, we will create a semi-active hetero-oligomer of PRMT1 to determine the catalytic role of PRMT1 oligomerization in regulating substrate binding and methylation. The proposed study will yield new non-radioactive assays of PRMT catalysis, provide molecular understanding of PRMT substrate specificity, and offer critical insight for designing specific PRMT1 inhibitors; b) Develop chemical probes to sort out the substrate specificity of PRMT1 in prostate cancer cells. The importance of PRMTs in prostate cancer pathogenesis is increasingly recognized. To reveal the function of PRMTs in the disease, we propose a series of unique chemical biology approaches to investigate the substrate specificity of PRMT1 in both androgen-dependent and androgen-refractory prostate cancer cells. First, we will prepare biotin-labeled PRMT1 to identify PRMT1- interactive proteins, from which substrate candidates will be determined. Second, we will design, synthesize and evaluate AdoMet analogs as chemical probes to investigate cellular substrates of PRMT1. Third, we will create new chemical probes for global mapping of arginine-methylated substrates in prostate cancer cells. The results of the proposed research will be essential for understanding the mechanism and the biological impact of PRMT-catalyzed methylation in gene regulation and signal transduction. Accomplishment of the proposed work will also provide new chemical tools for both basic PRMT biology research and facilitate the development of therapeutic agents for the treatment of carcinoma, cardiovascular disorders, and other diseases related to the deregulation of protein arginine methylation.